1. Field of the Invention
The present invention relates to an optical communications device which is used as an optical information transmitter, receiver, or transmitter-receiver in various optical communications systems.
2. Description of the Related Art
In general, in optical communications systems, it is expected that current consumption will be reduced by reducing the interval of time during which light is emitted at communications time. Thus, a reduction in the interval of time during which light is emitted is very useful for battery-powered terminal equipment in particular. A system involving such terminal equipment is an industrial instrumentation/control system such as an optical-fiber-based instrumentation system.
FIG. 1 is a block diagram of a conventional optical communications device which sends and receives optical information. This device is terminal equipment which is constructed from a communications LSI 2, a transmitter circuit 3, and a receiver circuit 4 to send and receive optical information over an optical fiber 5. That is, optical digital information sent over the optical fiber 5 from another optical communications device is converted into electrical information by an opto-to-electrical (O/E) converter 4A in the receiver circuit 4, and then applied to the succeeding communications LSI 2. The LSI performs predetermined processes, such as serial-to-parallel conversion, on the electrical information from the receiver circuit 4. At transmission time, on the other hand, electrical information produced by the LSI 2 is converted into optical information by the transmitter circuit 3 and then sent to another optical signal transmission device over the optical fiber 5.
FIGS. 2A, 2B and 2C are timing diagrams for use in explanation of the operation of the optical communications device shown in FIG. 1. More specifically, FIG. 2A shows original electrical pulse signals at the transmitting end, FIG. 2B shows optical pulse signals sent over the optical fiber 5, and FIG. 2C shows electrical pulse signals output from the O/E converter 4A at the receiving end. From these figures it is clear that the original electrical pulse signals at the transmitting end (FIG. 2A) are converted into equivalent optical pulse signals (FIG. 2B), that is, each electrical pulse signal is converted into a corresponding optical pulse signal such that its optical intensity level is proportional to the voltage level of the electrical signal. The optical pulse signals (FIG. 28) sent over the optical fiber 5 are converted into the equivalent electrical pulse signals (FIG. 2C) by the O/E converter 4A at the receiving end, that is, each optical pulse signal with an optical intensity level is converted into a corresponding electrical pulse signal having a voltage level proportional to that optical intensity level, so that the original electrical pulse signals from the transmitting end are recovered unchanged.
At this point, there is no difference in waveform between the original electrical signal to be sent and the corresponding optical signal being transmitted; thus, in order to emit light at the transmitting end, it is required to supply an optical emitting device, such as an optical emitting diode (LED), with a high current for an interval of time that is equal to the interval of time that each original electrical pulse signal (FIG. 2A) is at the high level.
In the industrial-instrumentation/control-system-oriented communications standard, "Field Bus", that is now in the process of being standardized, a signal in the Manchester coding form is sent at a rate of 31.25 kbps (31.25 kilobits per second). Here, in the Manchester coding, data is coded such that the positive- or negative-going transition takes place in the middle of each bit period, and the positive-going transition represents data "0", while the negative-going transition represents data "1". Therefore, it is required to supply the optical emitting diode with a current for optical emission during the half of the period of time (32 microseconds) required to transmit one bit of data. For example, supposing that the current required to emit light is 16 milliamperes, the current consumption when five bits of data are sent as shown in FIG. 2A will be 32 (microseconds).times.1/2.times.5 (bits).times.16 (milliamperes)=80 (microseconds).times.16 (milliamperes).
Thus, in the prior art, optical information sent from the transmitting end and electrical information to be transmitted to the receiving end have the same waveform and a high current flows through an optical emitting diode when the original electrical information is at a high level; thus, the problem with terminal equipment in which communications is frequently made is its power consumption. As a result, with battery-powered terminal equipment in particular required to reduce power consumption, it is difficult to use such a communications system as described above as it is necessary to frequently replace or recharge the battery.